It’s raining, it’s pouring

Flooding events across the North Island have resulted in fatalities, uninhabitable homes, and vital infrastructure in need of repair. What on earth is going on?

Auckland was recently overwhelmed by record breaking rainfall, with resulting floodwaters causing loss of life and widespread damage across the city. The extreme downpours have also caused landslips, road closures, and flooding in other parts of the North Island.

Where did all this rain come from?

The heavy downpour is said to have resulted from a storm in the north Tasman Sea linked to a source of moisture from the tropics. Meteorologists refer to this as an ‘atmospheric river.’ This isn’t the first time we have seen this type of weather event. Just last year an atmospheric river was responsible for a 1-in-120-year rainfall event in Nelson.

While this weather event manifested in extreme rainfall, there is a broader conversation to be had, about temperatures.

We know that the ocean heat content is increasing worldwide. We also know that the atmosphere can hold greater levels of moisture, at a rate of seven percent per degree Celsius warming. As sea temperatures around some parts of New Zealand sit at 3°C above normal, and 1°C above normal in regions to the north, it is likely that an additional 10 to 25 percent of additional moisture has been available for storms to effectively pick up and wring out over nearby land.

Climate scientists attempt to understand the influence of climate change on high rainfall events like that which we have just experienced (this is known as climate change attribution). To do this, computational models simulate a scenario where the climate is not altered and compare this with a simulation of a real event. The difference in outcomes is attributed to climate change. While determining the hand of climate change on this recent weather event may take some time, researchers have previously determined that flooding events in Canterbury in 2021 were 10 to 15 percent more intense, attributable to human induced climate change.

That explains the rain, but what about the flooding?

Auckland has a storm water system in place. When it rains, the untreated water flows through a network of underground pipes to streams and the sea. However, Auckland has buildings, car parks, and thousands of kilometres of roads, all constructed from impermeable materials. Our existing network of storm water pipes can only hold so much. Once that system is overwhelmed, the water has nowhere else to go, and so the streets turn into rivers.

What can we do?

As well as doing our bit to reduce global emissions, we need to adapt to our changing climate.

There is an opportunity to adapt our storm water management. You might think that we need bigger drains and larger pipes that are scaled for extreme weather events and our growing population. That might work, but there are other options too.

One suggestion you may have seen is the ‘sponge city’ concept. This involves managing storm water through increased infiltration, detention, storage, treatment, and drainage. Intentionally planning in permeable surfaces (through re-exposing streams, re-establishing wetlands, ensuring sufficient park spaces, sports fields, berms, etc) creates an environment with built-in capacity to cope with heavy rainfall while alleviating the pressure on our storm water network. In some instances, it may also involve uncovering natural waterways that have previously been sealed and paved over. And Aotearoa New Zealand is no stranger to this concept.

Auckland suburb, Stonefields, has been designed with ‘sponginess’ in mind. Within the development, a floodable park and wetland were incorporated, both designed to soak up water. Similarly, the Te Auaunga restoration in the Wesley area helped to reduce flooding risk to nearby homes while naturalising portions of the Te Auaunga Oakley Creek.

The outcomes?

Rainwater collects during heavy downpours and soaks back into the ground over time. Importantly, since the rainwater has somewhere to collect and soak into, the risk of flooding on nearby housing is reduced. We can also look at good examples overseas, such as Singapore, where they implemented more urban ponds, streams and greenspace following frequent flooding in the 60s and 70s. Despite regularly seeing 200mm of rainfall in a 24-hour period, they do not flood.

Are sponge cities the only way to futureproof?

Adopting a spongier urban landscape is just one of several actions that could be taken.

The flooding events highlighted many factors that need to be considered moving forward. Ten wastewater pumps stations in Auckland flooded as the heavy rains fell, damaging electrical circuits and pumps. Our wastewater infrastructure, and infrastructure in general, could be made more resilient. For example, pump stations could be elevated or relocated to drier grounds, roads could be made from porous cement, and green roofs could help with the reduction of rain runoff.

Beyond physical infrastructure, there is a need to develop a stronger understanding of environmental conditions, prior to allocating land for homes. Pockets of our population currently live in areas with known flooding, landslide or erosion risks. Following the recent flooding events, we may see increased pressure to prevent development in risk prone landscapes and, in some areas, to coordinate a managed retreat and relocate affected communities.

The Intergovernmental Panel on Climate Change tells us that globally, extreme precipitation events should be expected more frequently as warming continues.

The most recent flooding serves as a reminder of the need to reduce greenhouse gas emissions and slow the effects of climate change. Aotearoa New Zealand has an emissions reduction plan which lays out targets, and actions over the next 15 years.

It also challenges us to re-imagine the way that we design our cities and urban spaces now, and into the future.

With weather events continuing to unfold across the motu, we’ll share more resources on this topic as they become available.